Nonaqueous electrolyte batteries capable of charging and discharging, for example lithium-ion secondary batteries, are mainly used as power sources for electric vehicles such as hybrid electric vehicles and plug-in electric vehicles, which are rapidly spread in recent years. A lithium-ion secondary battery is manufactured by, for example, the following method. After an electrode group in which a positive electrode and a negative electrode are wound with a separator provided therebetween is produced, the electrode group is accommodated in a case of metal such as aluminum and an aluminum alloy. Subsequently, a lid is welded to an opening of the case, a liquid nonaqueous electrolyte is injected into the case via a liquid injection port provided in the lid, and then, a sealing member is welded to the liquid injection port, thereby manufacturing a battery unit. After that, initial charge and aging treatment are performed on the battery unit to obtain a lithium-ion secondary battery.
In the nonaqueous electrolyte battery, it is necessary to increase an energy density so as to extend a cruising distance of an electric vehicle. In addition, since acceleration performance is also required, it is necessary to reduce resistance so that high current can be charged and discharged and excellent input-and-output characteristics can be obtained. In order to improve both characteristics regarding the energy density and the resistance, a thickness reduction of a separator and an increase in porosity of the separator are being performing.
A self-supporting thin separator is made to have a high density so as to ensure sufficient tensile strength, and as a result, the porosity becomes small.
Another method of forming a thin separator, there is a method of directly forming a separator on an electrode by coating or the like. If coating is performed by using a gravure roll, a thickness reduction is possible, but there is a problem that the porosity is reduced. In addition, a separator obtained by forming a nonwoven fiber on an electrode by a melt blowing method, an electrospinning method, or the like may be subjected to a hot press after formation of the nonwoven fiber to increase density, so as to increase resistance to damage at the time of manufacturing a battery. Such an increase in density also reduces the porosity of the separator.
Therefore, there is a need to establish a method for achieving both the thickness reduction of the separator and the increase of the porosity of the separator.